1. Field of Invention
This invention relates to an apparatus and method for a master unit in a multidrop network to communicate to and from a plurality of remote units, using a plurality of host applications using half duplex polled protocols, through the use of time division multiple access techniques.
2. Scope of the Prior Art
In the prior art, in order to run multiple host applications to multiple modems in a multidrop network, it is common to use a single network channel for each application, thereby effectively resulting in a number of networks rather than a single network. Further, such an arrangement is clearly an inefficient use of leased lines and other equipment.
A common solution to this deficiency of the prior art is to use a single line with frequency division multiplexing. That is, a number of orthogonal carrier frequencies, one for each application, are transmitted over a single line to a plurality of remote units in a multidrop network. However, with such apparatus, the non-linearities of the communications line (most frequently, a telephone line), interfere with the co-existence of multiple carrier frequencies. This interference includes inter-modulation, cross-modulation and spillover between and among channels. Furthermore, a strong signal on one carrier frequency could suppress a weak signal on another carrier frequency.
Due to this interference, time-consuming engineering adjustment is required to install and maintain such a system. Such interference increases with an increasing number of co-existing carrier frequencies, thereby limiting the number of carrier frequencies which could be practically carried on a single line. For practical applications, no more than three carrier frequencies can be carried simultaneously on a telephone line.